Millisecond pulsars (MSPs) are characteristic by their stable rotations. It is possible to detect nanohertz gravitational waves by monitoring pulse times of arrival (ToAs) of an ensemble of the most stable MSPs. The success of this experiment strongly depends on the achievable timing precision. On short timescales, pulsar timing precision is limited by white noise. Radiometer noise and phase variation of integrated profiles induced by pulse-to-pulse variability, commonly referred to as jitter noise, are the main sources of white noise. Thus, single pulse studies of MSPs can provide us a fundamental limit on the achievable timing precision on short timescales.
We present radio observation of a millisecond pulsar PSR J0621+1002 using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The pulsar shows periodic pulse intensity modulations for both the first and the third pulse components. The fluctuation spectrum of the first pulse component has one peak of 3.0 ± 0.1 pulse periods, while that of the third pulse component has two diffused peaks of 3.0 ± 0.1 and 200 ± 1 pulse periods. The single pulse timing analysis is carried out for this pulsar and the single pulses can be divided into two classes based on the post-fit timing residuals （Figure 1）. We examined the achievable timing precision using only the pulses in one class or bright pulses. However, the timing precision improvement is not achievable.
Figure 1: Single pulse timing residuals (black dots) for PSR J0621+1002. The red bars are the ToA uncertainties.
